1. Field of the Invention
The present invention relates to a hydraulic control apparatus for controlling a hydraulically operated power transmitting system of a motor vehicle.
2. Discussion of the Prior Art
A known power transmitting system for a motor vehicle is adapted to transmit power from an engine to drive wheels of the vehicle, through a reversing gear mechanism hydraulically operated for establishing the reverse position for running the vehicle in the reverse position. For example, the power transmitting system uses a transmission having a reverse drive position as well as forward drive positions, or a continuously variable transmission equipped with a reversing device connected thereto. The power transmitting system of this type has a hydraulic circuit for applying a pressure to an appropriate hydraulic actuator for selecting or establishing the reverse position of the transmission or reversing device in response to an operation of a shift lever to the reverse position. An example of a hydraulic control device incorporating such a hydraulic circuit is disclosed in laid-open publication No. 64-49749 of unexamined Japanese Patent Application. If the shift lever is operated from a forward drive position to the reverse position through the neutral position during a forward running of the vehicle, the reversing gear mechanism is subject to an abrupt change in the load due to an automatic shifting action thereof to the reverse position, which may result in a considerable degree of shock to the power transmitting system, deteriorating the driving comfort of the vehicle.
In the light of the above drawback, the provision of a reverse inhibit valve in a hydraulic line leading to the hydraulic actuator for the reversing gear mechanism is proposed as disclosed in unexamined Japanese Patent Application No. 63-34598. For operating the reverse inhibit valve, solenoid-operated valve means is provided for generating a hydraulic signal or pilot pressure to be applied to the reverse inhibit valve, so that the reverse inhibit valve is operated to the reverse inhibit position for inhibiting the reversing gear mechanism from being placed in the reverse position if the shift lever is moved to the reverse position while the vehicle is running in the forward direction.
In a known hydraulic control arrangement for the power transmitting system using a reverse inhibit valve as indicated above, the solenoid-operated valve means generally uses a single solenoid-operated valve for producing a pilot pressure in response to an electric signal generated from an electronic control device. If this solenoid-operated valve fails to normally function due to sticking of its valve spool, for example, the reverse inhibit valve is kept in the reverse inhibit position and is prevented from being brought to the non-inhibit position. In this event, the reversing gear mechanism cannot be placed in the reverse position to move the vehicle in the reverse direction even while an electric signal is applied to the solenoid-operated valve to place the reversing gear mechanism in the reverse position.
In the meantime, the power transmitting system has two or more hydraulic control modes which are established by respective pilot pressure generating solenoid-operated valves operated according to electric signals from the electronic control device. If a relatively large number of the hydraulic control modes are available, an accordingly large number of pilot pressure generating solenoid-operated valves are required, causing the hydraulic control arrangement to be accordingly complicated and large-sized.